The present invention relates to optical networks. In particular it relates to method and apparatuses for improving optical networking properties by broadening the passband response of network components comprising arrayed waveguide gratings.
Loss reduction in optical networks is of major concern because it reduces the need for optical signal amplification. A flattened passband of any network component is a general aim of optical network traffic as it reduces the strict requirements on wavelength control and allows cascadability of components without affecting optical transmission properties. Therefore both low losses and flat passbands are important factors to reduce cost in optical networks.
In FIG. 1 an Arrayed Waveguide Grating, further referred to herein as AWG is drawn schematically. This component can be used to separate a set of wavelengths impinging on a single input waveguide port. Each wavelength is transferred to a separate output waveguide. Reversibly, the AWG can take a number of wavelengths entered at respective input ports and combine them all on one output port. The component can thus be used as a wavelength demultiplexer and as a multiplexer.
FIG. 2 shows an example of the calculated transfer function of an AWG as disclosed in xe2x80x98Smit, M:K., van Dam,C., xe2x80x9cPHASAR-Based WDM-Devices: Principles, Design and Applicationsxe2x80x9d, IEEE Journal of Selected Topics in Quantum Electronics, vol. 2, pp.236-250, 1996xe2x80x99 from an input channel to two adjacent output channels with a wavelength separation of 1.6 nm. The shown transfer function is generally obtained for a prior art AWG, improved for a channel to channel crosstalk smaller than xe2x88x9240 dB. For this standard transfer function, the xe2x88x921 dB bandwidth of a channel is approximately 30% of the channel spacing.
To reduce the sensitivity of a communications system for wavelength drifts either in transmitter lasers or in an AWG, it would be advantageous as a general aim of the present invention to increase the width of the xe2x88x921 dB passband relative to the channel spacing, i.e., to flatten the transmission function.
For broadening said passbands in prior art, amongst others, multimode receiver waveguides at the output of the AWG are used. This method, however, only works when the AWG is used as demultiplexer, immediately followed by the detector diodes.
Therefore, an object of the present invention is to provide an improved method and apparatus for a broadening the passband of optical network components having one or more Arrayed Waveguide Gratings.
These objects of the invention are achieved by the features stated in enclosed independent claims. Further advantageous arrangements and embodiments of the invention are set forth in the respective subclaims.
According to a first aspect the present invention discloses a method for improving the passband of an optical network device -or apparatus- which includes an arrayed waveguide grating. The essential feature is the step of generating a controllable and thus somehow xe2x80x98dynamicxe2x80x99 intensity field profile with a controllable beating pattern in a multimode superposition of different modesxe2x80x94as e.g., a fundamental mode and a first mode, or higher mode, respectively or a superposition of higher modes onlyxe2x80x94of an input wavelength entering the input site of said apparatus, whereby said beating pattern is controlled in a fixed, predetermined way for improving the mode overlap in a receiver waveguide associated with an output side of said apparatus in order to achieve a wavelength-dependent coupling performance when coupling said multimode superposition into said receiver waveguide. For simplicity, throughout the following description it is referred to only exemplarily to the TE0 mode as the fundamental mode and to the TE1 mode as a higher order mode.
This is achieved for example in a multiplexer component by a symmetrical arrangement of multiple converter units at both, the input section of the input star coupler and the output section of the output star coupler of the multiplexer. Thus, in an 8:1 multiplexer, only 8 input converter units and one output converter unit are required.
As an advantage, it is possible to increase the xe2x88x921 dB passband width to channel spacing ratio from 30% to approximately 60% with a negligible loss penalty, i.e., less than 0.4 dB compared to the AWG cited above. The method is applicable to optical network apparatuses as both, multiplexers and demultiplexers, and for Mxc3x97N coupler connecting M input lines to N output lines.
In a prior art AWG the field of a transmitter waveguide is imaged by the waveguide array onto the output side of the second star coupler. With changing wavelength, the position of the image shifts along the star coupler output, from one output waveguide to the next, etc.
The control of said beating pattern advantageously comprises the steps of generating said intensity field profile at the transmitter and receiver waveguides such that for a wavelength range around the AWG channel wavelengths the spatial shift in the position of the peak of the dynamic field profile counterbalances the wavelength dependent shift of the image of the field of the transmitter waveguide. This assures that the coupling from transmitter to receiver waveguide stays high over this wavelength range resulting in a flattened passband. Due to the periodicity of the dynamic field profile, the counterbalancing effect, and thus the passband -flattening, can be made to re-occur for all channel wavelengths.
Advantageously, said step of generating a dynamic field profile is achieved by a superposition of a fundamental mode and a higher order mode, as e.g., TE0 mode and a TE1 mode, in a bimodal transmitter waveguide in the input section of an AWG according to the present invention, and said periodic movement is generated by imposing a wavelength dependent phase shift of an integer multiple of 2 B for each wavelength change of the size of the channel spacing.
According to a second, preferred embodiment, a further aspect of the present invention is disclosed. According to said second aspect said step of converting monomode light, of each of said input waveguides of said AWG-comprising apparatus, into a respective superposition of multiple modes comprises the step of making a power ratio between said fundamental mode and said higher order mode specific for each input wavelength. Here, a lower number of converter units is required. In a 8:1 multiplexer, for example, there is needed just one converter unit at its output. A 1:8 demultiplexer can be obtained by solely reversing the AWG apparatus. For purposes of generalization only it should be mentioned that it would also be possible to provide the demultiplexer apparatus with 8 converter units at its output and none of them at its input. This, however would be a waste of resources.
The key advantage of these improved AWG designs lies in the fact that they provide a method to obtain a flattened passband without the introduction of 3 dB extra loss, as opposed to other broadened passband designs.